Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. Thermal barrier coating systems capable of satisfying the above requirements have generally required a metallic bond coat deposited on the component surface, followed by an adherent ceramic layer that serves to thermally insulate the component. In order to promote the adhesion of the ceramic layer to the component and inhibit oxidation of the underlying superalloy, the bond coat is typically formed from an oxidation-resistant aluminum-based intermetallic such as a diffusion aluminide or platinum aluminide, or by an oxidation-resistant aluminum-containing alloy such as MCrAlY (where M is iron, cobalt and/or nickel).
Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and electron beam vapor deposition (EBPVD) techniques, have low coefficients of thermal expansion, and when processed properly show good adhesion to substrate materials. In order to increase the resistance of the ceramic layer to spallation when subjected to thermal cycling, the prior art has proposed various improved coating systems, with considerable emphasis on ceramic layer microstructures having enhanced strain tolerance as a result of the presence of porosity, microcracks and segmentation of the ceramic layer. Thermal barrier coating systems employed in higher temperature regions of a gas turbine engine are typically deposited by physical vapor deposition (PVD) techniques that yield a grain structure that is able to expand without causing damaging stresses that lead to spallation.
The bond coat is also critical to promoting the spallation resistance of a thermal barrier coating system. As noted above, bond coats provide an oxidation barrier for the underlying superalloy substrate. Conventional bond coat materials contain aluminum, such as the diffusion aluminides and MCrAlY alloys noted above, which enables such bond coats to oxidize and grow a strong adherent continuous aluminum oxide layer (alumina scale). The aluminum oxide layer chemically bonds the ceramic layer to the bond coat, and protects the bond coat from oxidation and hot corrosion. Though bond coat materials are particularly alloyed to be oxidation-resistant, oxidation inherently occurs over time at elevated temperatures, which gradually depletes aluminum from the bond coat. In addition, aluminum is lost from the bond coat as a result of interdiffusion into the superalloy substrate. Eventually, the level of aluminum within the bond coat is sufficiently depleted to prevent further growth of oxide, at which time spallation may occur at the interface between the bond coat and the aluminum oxide layer or the interface between the oxide layer and the ceramic layer.
In addition to depletion of aluminum, the ability of the bond coat to form the desired aluminum oxide layer can be hampered by the diffusion of elements from the superalloy into the bond coat, such as during formation of a diffusion aluminide coating or during high temperature exposure. Oxidation of such elements within the bond coat can become thermodynamically favored as the aluminum within the bond coat is depleted through oxidation and interdiffusion. Elements such as nickel, chromium, titanium, tantalum, tungsten, molybdenum and hafnium can increase the growth rate of aluminum oxide and form voluminous, nonadherent scales that may be deleterious to adhesion of the ceramic layer.
From the above, it is apparent that the service life of a thermal barrier coating is dependent on the bond coat used to anchor the thermal insulating ceramic layer. Once spallation of the ceramic layer has occurred, the component must be refurbished or scrapped at considerable cost. Therefore, it would be desirable if further improvements were possible for the service life of a thermal barrier coating system.